Ideal amounts of spellpower for discipline is basically "as much as you can get". I run 2700 raid-buffed spellpower in my disc set, and I'd love to push that up +300 or +400. Spellpower and crit rock for Disc, in as much quantity as you can get.
As far as the metagems, I believe I did an analysis of them in the Compendium OP, but here's another one:
Insightful Earthsiege Diamond
21 intellect, raid-buffed = 23 (holy) or 26.5 (disc)
Proc = 600 mana, 15 sec ICD, 5% chance to proc. Total of ~ 75Mp5.
Ember Skyflare Diamond
2% intellect matches 26.5 intellect @ 1325 raid-buffed intellect pre-meta, or roughly BiS Ulduar gear across the board. I run ~ 1325 *with* meta right now, and I gem heavily for intellect. Easier to get to this point if you have
[Pandora's Plea] or similar, of course.
25 spellpower, flat. No scaling.
So you can match the static intellect with the 2% intellect fairly "trivially", assuming you have good Ulduar gear. However, there's no world where 25 spellpower > 75 Mp5, even for Disc. If you find you absolutely 100% never run out of mana, then by all means try it out. Otherwise, IED is the best metagem, hands down.